In recent years, applications for hydroforming have been growing—particularly in the field of auto parts. The advantages of hydroforming are that it is possible to form an auto part, which used to be made from several press-formed parts, from a single metal tube, that is, combine parts and thereby reduce costs, and reduce the number of welding locations and thereby lighten the weight.
On the other hand, with hydroforming, it is necessary to control the two parameters of the internal pressure and axial pushing action so as to form the part. If the load path of these two parameters (hereinafter referred to as simply the “load path”) is unsuitable, the metal tube may crack in the middle of being worked, buckling or wrinkles may end up remaining, and other working defects may be caused.
A general example of the load path is shown in FIG. 1. First, it is comprised of stage 1 of raising only the internal pressure (to seal the tube ends, sometimes a slight axial pushing action is also given), stage 2 of applying the internal pressure and an axial pushing action in a broken line pattern, and stage 3 of raising only the internal pressure for sharply forming the corners (with shapes with no corners, sometimes this is omitted, while to secure a seal of the tube ends, sometimes a slight axial pushing action is also given).
Among these, finding a suitable path for stage 2 consumes the most effort and has relied heavily on the skill of the hydroforming workers.
From the above background, recently several methods for simply obtaining the load path have been proposed.
For example, Patent Document 1 discloses the method of preparing in advance a crack limit line and a wrinkle limit line and selecting a load path between the two limit lines. However, in actuality, it is difficult to prepare these two limit lines. Usually, a large number of experiments and trial and error in analysis of numerical values are required. Further, the limit lines are often broken lines. If so, the number of parameters for determining the broken lines becomes greater and therefore tremendous labor becomes necessary for the trial and error.
Further, Patent Document 2 proposes a method of performing FEM analysis and monitoring the surface area, volume, or thickness of the metal tube to find the suitable load path. The information monitored here can be monitored by FEM analysis, but cannot be monitored during actual hydroforming.
As opposed to this, Patent Document 3 of the present inventors proposes a working method and working system embedding sensors for measuring the stress or strain in the actual hydroform mold and deriving the suitable load path from that information.
However, in the above prior arts, in each case, at stage 2 in the load path (FIG. 1), paths are employed raising the internal pressure as well along with the increase in axial pushing action. For this reason, at least two parameters, for example, the internal pressure and axial pushing amount or the axial pushing amount and inclination have to be determined. This becomes extremely complicated. Further, when stage 2 is a broken line, the parameters increase more, so finding a suitable load path becomes further difficult.